Sunlight is a general term for the electromagnetic radiation emitted by the sun (also known as Solar radiation). It provides both light and heat for the many creatures that inhabit the Earth, but also serves as the source of energy that plants use to perform photosynthesis. In addition, sunlight is an abundant natural resource that humans can capture and convert into other useful forms of energy, such as electricity, using a variety of technologies.
Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one "spot" on the Earth's surface varies according to these factors:
- Geographic location
- Time of day
- Local landscape
- Local weather
Because the Earth is round, the sun strikes the surface at different angles ranging from 0º (just above the horizon) to 90º (directly overhead). When the sun's rays are vertical, the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year.
The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth's surface receives a little more sunlight. The Earth is nearer the sun when it's summer in the southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.
The 23.5º tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other six months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.
Countries like the United States, which lie in the middle latitudes, receive more sunlight in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more sunlight in June than they do in December.
The rotation of the Earth is responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon when the sun is at its highest point. On a clear day, the greatest amount of sunlight reaches the surface around solar noon.
Diffuse and Direct Solar Radiation
As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by the following:
- Air molecules
- Water vapor
- Forest fires
This is called diffuse solar radiation. The sunlight that reaches the Earth's surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.
Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface, or as total radiation on a surface tracking the sun.
- Solar Radiation Basics U.S. Department of Energy. Accessed July 25, 2010.